The game arrived in the Repair Station
completely dead. A check of the monitor found it to be working. Pulling
ROM 'H'
did not
produce the standard RAM bars. The CPU and crystal tested
fine. There
was output on pin 18 of the 18 pin connector (composite video signal)
but still
no picture. Checking the board with a logic probe revealed that
pin 15 of
the 74166 located at C4 was not pulsing. This was the reason the sync signal was dead and working upstream
from C4 the culprit turned out to be a shorted 7402 located at D6 on
the mother
board. Once the chip at D6 was replaced the RAM bars could now be
seen.

After reinstalling ROM 'H' and powering up
the
game two problems were evident. There was no sound and there were two
lines of dots on the screen which interfered
with game play. The dots blocked shots from the base as well as
from the Invaders. After some analysis of the PCB I
determined I had two bad RAM chips. RAM chip failure seems
to be very common on the Midway 8080 boards.

I decided to tackle the sound issue first.
Checking all the LM3900 op-amps revealed them to be fine. The 4006 chip
at N5
also tested good. The previous checks pointed to the LM1877 amplifier
being bad but
in order to check that I ran my finger over the pins on the solder side
of the
board. Doing this should produce static or some audio output but in
this case
there was only silence. To confirm this finding I checked the power pin
of the amp - pin 14 - with my digital multimeter and foud it was dead
shorted to both output pins 2 and 13. I replaced the LM1877 and
transferred the heat sink from
the old chip to the new one..

Here
is a picture with one of the RAM chips replaced and one removed with
the holes cleaned up. Normally I would socket a repair like this
but at the time of the repair I found myself out of sockets!

This picture shows both
of the ram chips replaced. Notice MM5280 is compatible with
UPD411. Suitable other replacement chips are TMS4060, MM9060 and
D2107C.

After
all the repairs were done I reinstalled the PCB and turned the game on.
No more dots and the Invaders were thumping away! All sounds checked
good. Let the invasion begin!!!